Magnetic field sensors may be used in many applications, such as magnetic compassing, ferrous metal detection, and current sensing. Magnetic field sensors may detect a magnetic field and its variations in many instances, including machine components, the earth's magnetic field, underground minerals, and electrical devices.
One type of a magnetic field sensor is an MR sensor. The MR sensor is typically composed of magnetoresistive material and formed using integrated circuit fabrication techniques. The magnetoresistive material may be permalloy, which is a ferromagnetic alloy containing nickel and iron.
The magnetoresistive material may be arranged in thin strips that are several times longer than they are wide. Each magnetoresistive strip may have a long axis, which may be referred to as an “easy” axis, and a short axis. A magnetoresistive strip may be magnetized in a particular direction, such as the direction of the easy axis, during the fabrication of the magnetoresistive strip. If the magnetoresistive strip senses an external magnetic field, the magnetization direction of the magnetoresistive strip may rotate away from the initial magnetization direction.
When a current is applied to the magnetoresistive strip, the resistance of the magnetoresistive strip may depend on angle between the direction of current flow and the direction of magnetization. A magnetization direction parallel to the current flow direction may result in maximum resistance through the magnetoresistive strip and a magnetization direction perpendicular to the current flow direction may result in minimum resistance through the magnetoresistive strip.
If the magnetoresistive strip senses a magnetic field that alters the magnetization direction of the magnetoresistive strip, the resistance of the magnetoresistive strip changes, which causes the voltage drop across the magnetoresistive strip to change as well. This change in the voltage drop across the magnetoresistive strip may be measured. The voltage change may indicate a change in the strength and/or direction of the magnetic field sensed by the magnetoresistive strip.
The magnetoresistive strip consists of many magnetic domains. The highest sensitivity is attained by aligning all the domains in a preferred direction. However, the alignment of the domains, and hence the sensitivity of the MR sensor, may be upset after manufacture due to thermal decay and by the presence of powerful magnetic fields near the magnetoresistive material of the MR sensor. Therefore, the sensitivity of an MR sensor may need to be set to the initial sensitivity prior to measurement.
One option for setting the sensitivity of an MR sensor may be to use large external magnets. However, large external magnets may not be practical when the MR sensor has already been packaged into a system or when several sensors within a single package must be magnetized in opposite directions.
Another option for setting the sensitivity of an MR sensor may be to wrap individual coils around each MR sensor. However, individual coils may be expensive and may be unable to generate the large magnetic fields required to set the sensitivity of MR sensors.
Yet another option for setting the sensitivity of an MR sensor may be to use a current strap, which may be referred to as a Set-Reset strap. A DC current pulse is typically applied to the Set-Reset strap to set the sensitivity of the MR sensor. The design and operation of a Set-Reset strap is discussed in U.S. Pat. No. 5,247,278 to Bharat B. Pant and assigned to the same assignee as the current application. U.S. Pat. No. 5,247,278 is hereby fully incorporated by reference.
Set-Reset straps may provide a more efficient means of setting the sensitivity of an MR sensor than external means, such as external magnets or individual coils, which may be larger and more expensive. Moreover, because the Set-Reset strap may be formed on the same substrate as the MR sensor, the Set-Reset strap may be located closer to the magnetoresistive strips than other options for setting the sensitivity of the MR sensor. This may allow for less energy to be used in setting the MR sensor.
Additionally, the Set-Reset strap may also be used to eliminate offset created in the MR sensor. The offset may be the result of imperfections in the magnetoresistive strips of the MR sensor. To eliminate the offset, a first DC current pulse may be applied to the Set-Reset strap in one direction. The first DC current pulse may be referred to as a SET current pulse. Then a second DC current pulse with a magnitude substantially the same as the magnitude of the SET current pulse may then be applied to the current strap in a direction substantially opposite to the direction of the SET current pulse. The second DC current pulse may be referred to as a RESET current pulse. Voltage measurements taken after both the SET current pulse and the RESET current pulse may be used to eliminate the effects of offset from the MR sensor output signal.
MR sensors are used in many consumer and industrial applications. Generally, MR sensors are small in size, reliable, manufacturable, and cost effective. However, the resolution of these MR sensors is limited by sensor noise and a large bridge offset voltage as compared with the useful signal. The sensor's noise is dominated by the Johnson noise of the resistive sensing elements and is characterized as 1/f noise, where f is the frequency. Since the useful signal of the sensor is in the DC domain, any DC signal conditioning applied to the sensor output will contain the noise components as well. Additionally, the amount of amplification that can be applied is limited by the bridge offset voltage. These factors limit the sensor's resolution.
Furthermore, the sensitivity or the efficiency of the sensor to translate the magnetic field into a voltage is proportional to the voltage applied to the sensor. Any attempt to operate the sensor at a low voltage to conserve energy will compromise the resolution due to a low signal-to-noise ratio. Accordingly, there is a need for a magnetometer that has a higher resolution, that consumes less power, and that is still small enough for use in consumer and industrial applications.